Evaluation of groundwater recharge and flow in crystalline terrain in the Matlala Batholith, Limpopo, South Africa
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Date
2021-11
Authors
Maphala, Londolani Hope
Journal Title
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Publisher
University of the Free State
Abstract
Communities residing near the Matlala Batholith are dependent on the groundwater resource for their water supply. The Matlala wellfield occurs near the south-eastern perimeter of the batholith outcrop and may in future supply local residents with water for drinking and domestic purposes. However, the groundwater level response to rainfall and the groundwater flow patterns within the study area are not well understood, and the risk of aquifer depletion exists if over-abstraction from the aquifer takes place. It is therefore important to understand the underlying aquifer system in terms of recharge and groundwater flow to allow determination of a sustainable abstraction rate. As a first step towards determining a sustainable yield for the wellfield, information on the recharge of the aquifer system is required. In this study the groundwater recharge to the Matlala Batholith and surrounding gneissic host rock is estimated by evaluating historical groundwater level and chemistry data. However, the available historical data is sparse and contain large temporal and spatial gaps. This lack of complete, comprehensive, and recent data was a severe limitation to the recharge investigation. Despite the limitations associated with data scarcity, three methods were used to estimate the groundwater recharge in the study area from the available data, namely the chloride mass balance (CMB), water level fluctuation (WTF) and cumulative rainfall departure (CRD) methods. For all three methods, the estimated recharge values varied significantly for the boreholes drilled around the Matlala Batholith. The large variations in the estimated recharge rates for the different boreholes indicate that different recharge conditions occur at these boreholes. The recharge rates are most likely affected by the depth of weathering and fracturing of the regolith in the vicinity of the different boreholes. The average recharge rates estimated with the physical methods (WTF and CRD) were very similar (3.39% and 3.76% of the rainfall received) but differed from the estimate obtained with the CMB method (1.20% of the MAP). However, all three methods indicate low average recharge rates of well below 5%, in line with the results obtained during previous studies. These low recharge estimates should be used when calculating the sustainable abstraction rate from the aquifer system. A way forward for future investigations at the Matlala wellfield includes: performing pumping tests on the high-yielding boreholes in the wellfield to investigate the flow regimes in the aquifers and to determine the aquifer hydraulic parameters, calculating sustainable abstraction rates using the results of the pumping tests, and developing a numerical model for the Matlala wellfield to investigate different pumping scenarios for future water supply purposes.
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Keywords
Dissertation (M.Sc. (Geohydrology))--University of the Free State, 2021, Groundwater -- South Africa -- Limpopo Province -- Matlala Batholith, Groundwater monitoring network, Crystalline aquifers